Nomenclature of amphiboles: Additions and revisions to the International Mineralogical Association's amphibole nomenclature

The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 a.f.p.u. (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of B(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which, here, is changed slightly, but significantly, from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized

Ionic radii

Definition and Assumptions An ion is an atom with an electrical charge, achieved either by gaining or losing one or more electrons. The ionic radius of the ion (rion) of an atom (either a cation or anion) is a measure of the size of a spherical ion. The ionic radius is similar to but different from the atomic radius for the ionic size is dependent on the distribution of its outermost electrons and is inversely proportional to the effective nuclear charge experienced by ions. It is calculated from the internuclear distance between a cation and a neighboring anion in a lattice. Ionic radii are typically reported in picometers (pm, 1 × 10−12 m) or in the older literature as Angstroms (Å), where 1 Å = 100 pm. A typical range of ionic radii is 25–170 pm for four to eightfold coordination (see Table 1).N/